The invention relates to a method of operating an internal combustion engine with a six-stroke process involving the following sequence of events:
1st stroke: air intake into the combustion chamber,
2nd stroke: compression of the air and injection of a first fraction of gasoline fuel into the combustion chamber,
3rd stroke: first working stroke after first ignition of the mixture, said first ignition being initiated by an ignition device,
4th stroke: new compression of the contents of the combustion chamber,
5th stroke: second working stroke after second ignition of the fuel-air mixture contained in the combustion chamber,
6th stroke: expulsion of the exhaust gases from the combustion chamber,
a second fraction of gasoline fuel being injected prior to the second ignition.
DE 34 06 732 A1 describes a working process for reciprocating internal combustion engines in which the fuel-air mixture is ignited either through an ignition device or by means of compression ignition. The working cycle process is carried out in six strokes and involves the following events:
1st stroke: intake,
2nd stroke: compression and injection of a fraction of gasoline,
3rd stroke: first working stroke,
4th stroke: new compression of the exhaust gas still containing free oxygen and injection of a second fraction of gasoline,
5th stroke: second working stroke,
6th stroke: expulsion.
This method is described both for the diesel engine in which the fuel is self-ignited and for the Otto-cycle internal combustion engine in which the fuel is ignited through an ignition device.
DE 33 17 128 A1 describes an internal combustion engine operating on a six-stroke cycle. During the intake stroke, a fuel-air mixture is admitted into the cylinder and is compressed during a first compression stroke. After ignition of the compressed fuel-air mixture, an expansion stroke occurs. In a subsequent second compression stroke, the combustion products are compressed. At the end of the second compression stroke the in-cylinder contents are ignited by a spark plug. As a result of the expansion of the compressed combustion products, a second expansion stroke comes next, finally followed by an exhaust stroke during which the combustion gases are driven out of the cylinder.
In the known six-stroke internal combustion engines, diesel fuel is auto-ignited or gasoline is spark-ignited at the beginning of the first and of the second working stroke.
Internal combustion engines operating at least partially on the homogeneous charge spark ignition of gasoline require high charge temperatures and high amounts of residual gas to ensure ignition of the air-fuel-residual gas mixture.
AT 3.135 U discloses a method of operating an internal combustion engine with gasoline fuel relying for operation on both compression ignition and spark ignition. At part load, the internal combustion engine is operated in the compression ignition mode, a homogeneous fuel-air mixture being produced in the combustion chamber and combustion being initiated by compression ignition of said fuel mixture. The process known as HCCI combustion (Homogeneous Charge Compression Ignition) has particular advantages with respect to the production of emissions. AT 5.140 U describes a method by means of which auto-ignition of the homogeneous fuel-air mixture may be controlled by the residual gas content in the combustion chamber.
At low speed and load in particular, the exhaust temperature is no longer high enough to heat the charge to an extent sufficient to ensure auto-ignition, even if the amount of residual gas is extremely high. Moreover, at low speed, the longer cycle times negatively affect the charge temperatures as there is more time available for heat transfer.
It is the object of the invention to avoid these drawbacks and to develop a method of operating an internal combustion engine by means of which high efficiency on the one hand and low emissions on the other hand may be achieved.
This is achieved in accordance with the invention in that the second fraction of gasoline is injected during the third stroke, preferably during the second half of the third stroke, that an at least almost homogeneous fuel-air mixture is formed in the combustion chamber prior to the second ignition and that the second ignition occurs by compression ignition of said homogeneous fuel-air mixture.
There is thereby preferably provided that the fuel-air mixture formed during the second stroke is lean and stratified and is provided with high excess air with an air/fuel ratio of xcex greater than 1.5. The excess air makes certain that there will still be enough oxygen available for the second combustion. Specifically when very small quantities of fuel are injected, the lean stratified combustion results in very low NOx emissions. The temperature of the burned gas is relatively low but high enough to ensure secure compression ignition in the second combustion cycle. It is particularly advantageous when the combustion taking place during the fifth stroke subsequent to the second ignition is controlled by adjusting the quantity of the first fraction of gasoline fuel injected during the second stroke. By simply varying the quantity of the first injection or by changing the amounts apportioned to the first and second injection the combustion of the combustion gas and, as a result thereof, the starting condition for compression ignition may be changed or adjusted.
Moreover, stratified combustion has the advantage to be very efficient.
But homogeneous auto-ignition is also characterized by good efficiency. More specifically as far as NOx and soot emissions are concerned, compression ignition provides substantial advantages. The fact that possible soot or hydrocarbon emissions resulting from the first working cycle are used for further combustion and are burned as a result thereof positively affects emissions.
In a particularly preferred variant of the invention there is provided that adjustment in pressure is carried out in the region of transition between the third and the fourth stroke. The regulation of the level of pressure in the combustion chamber may be used for controlling auto-ignition. It is particularly advantageous when the pressure is adjusted by temporarily opening at least one lift valve, preferably one intake valve.
A first cycle consisting of the sixth and of the first stroke thereby serves for gas exchange, meaning for driving out exhaust gas, and for the intake of a fresh charge. The second cyclexe2x80x94second and third strokexe2x80x94consists of compression, spark-ignited, lean stratified combustion (SCSIxe2x80x94Stratified Charge Spark Ignition) and expansion and forms the first working cycle. The lean exhaust gas resulting from this first working cycle is not driven out but remains inside the cylinder where it serves as a charge for the following third cycle (fourth and fifth stroke). Said third cycle is the second working cycle and is characterized by a homogeneous compression ignition (HCCI). The expulsion of exhaust gases of the first cycle follows.
The low loss in charge exchange and the combination of the two combustions SCSI and HCCI as well as the fact that this six-stroke working method provides for two working cycles for every charge exchange cycle guarantee high consumption potential and the lowest possible emissions.